Hitherto, as a method of constructing radio receivers, a heterodyne system or a direct conversion system has been used usually. In the heterodyne system, a high-frequency signal which was received by the receiver through an antenna is converted in frequency into an IF (Intermediate Frequency) signal using a super heterodyne system, and after the IF signal was amplified by an IF amplifier, it is detected to provide an audio signal. Whereas in the direct conversion system, the received high-frequency signal is converted directly to a base-band signal comprising two axial components of in-phase and orthogonal phase, and after the directly converted signal was amplified adequately, it is demodulated. The direct conversion system has the advantages that it can avoid image disturbance which would be produced in the heterodyne system and it can demodulate an FSK (Frequency Shift Keying) signal with a simple construction if a dedicated LSI (Large-Scale Integration) circuit is utilized. However, in order to demodulate the AM signal and the FM signal, a relatively complicated and specially designed circuit is needed in the direct conversion system, because for the demodulation of the AM signal .sqroot.I.sup.2 +Q.sup.2 should be obtained and for the demodulation of the FM signal I(dQ/dt)-Q(dI/dt)should be obtained (I and Q represent the two axial components of in-phase and orthogonal phase, respectively), and therefore respective amplifiers for these two axial components are required to have equal amplification degree and equal quantity of phase shift and also it is required to provide a differentiator with high precision. These requirements are not so strict to an amplifier and demodulator for the FSK signal because the FSK system only requires that information should be discriminated as two values (this means that the system can be constructed with an LSI). However, since a base-band amplifier for the FM signal is required to be considerably strict, an attempt to construct with an LSI an FM signal receiving circuit according to the direct conversion has not been put into practice up to now.